The problem of tracking and recovering multiple sinusoids from an optic multiple-wavelength signal is of high practical importance. For example, this problem arises in the non-invasive measurements of various blood parameters, e.g., blood oxygenation, that employ the detection of light transmitted or reflected from the location on the patient's body under measurement, and indication of the presence of various blood constituents based on known spectral behaviors of these constituents. Most of these techniques utilize a measurement optical device or probe attached to the patient's finger, which includes an optical assembly including a block of light emitting devices (e.g., LED) for irradiating the finger with light, and a photodetector (PD) for detecting the finger's light response. One of the conventional devices of the kind specified, such as a pulse oximeter, which is the generally accepted standard of everyday clinical practice, provides for measuring enhanced optical pulsatile signals caused by the changes in the volume of a blood flowing through a fleshy medium (e.g., finger). In conventional pulse oximeters as well as in other plethysmographic devices, an optical, amplitude-modulated light passes through the investigated sample (e.g., finger) at several (usually 2–3) different carrier wavelength (channels). Thereafter, a relationship between the intensities of the corresponding light responses is measured for determining the corresponding blood parameters. This technique utilizes the fact that different wavelength optic signals (channels) are transmitted differently, when passing through the sample, in accordance with the scattering and/or absorbing properties of the blood. In such techniques, the intensity of the signal obtained from the blood measurements (e.g., the intensity of photo-plethysmographic signal (PPS)) for the particular optical wavelength is obtained by means of a modulation-demodulation (M-D) procedure. For this purpose, the light emitting devices are switched on and off at a presumed rate (e.g., about 3–10 kHz), providing thereby a sequence (train) of light pulses for modulation of the corresponding carrier waves. The total pulsatile optical signal is detected by the photodetector and demodulated. The wavelength-related intensities are then derived from the demodulated optical signals.
In such conventional techniques, LEDs usually operate in one of two regimes, serial or parallel. In the serial mode regime, the operation of LEDs is separated in time. On the other hand, in the parallel mode regime, the LEDs are activated concurrently, but with the diverse modulation rates, or, in other words, the LED signals are separated in the frequency domain. The demodulation procedure used for separating different optic channels from each other is usually based on employing a bank of frequency-selective filters arranged either in cascade or parallel configurations.
More specifically, in the serial operation mode regime, each LED oscillates over a certain, relatively long period (including about 10–30 on-off LED ignition cycles), while the other LEDs are kept inactive over this period. An output signal, sensed by the photodetector, is then demodulated and associated with the corresponding wavelengths. Such a procedure is repeated for the next LED, et cetera.
In fact, the complexity of the modulation-demodulation technique rapidly increases with the number of the desired optical channels. Because of this, a majority of the commercial oximeters utilize few, e.g., 2–3 wavelengths. When such a relatively small number of optical channels is used, one can conduct PPS measurements for different optic channels in series, one by one. In such a case, as described above, each LED is activated serially over a certain period, used for demodulation, thus outputting a modulated signal related to the particular wavelength.
However, there are several critical points behind the serial mode regime. As the number of channels increases (e.g., close to ten), then the serial order comes into conflict with the required access rate. In other words, the physical and chemical processes, e.g., aggregation of red blood cells, occurring in the measured sample can be different for the time periods corresponding to the activation of different LEDs.
One of the shortcomings associated with the serial operation mode regime is the phase shift between the optical channels. Thus, the train of pulses (provided by a generator for LEDs' activation) switches between different LEDs by means of a multiplexer (MUX) such that only a single LED operates at a time. To this end, the longer LED operation period the higher delay between the different channels.
On the other hand, decreasing periods of LED activity restricts the demodulator selectivity and accuracy. So, in the serial mode, the demodulator selectivity and accuracy are in contradiction with the measuring process simultaneity.
Moreover, the serial mode regime yields various problems in signal processing. In this connection, it should be noted that the signal obtained from the blood measurements is usually affected by the electrical 50 Hz interference, lamp-induced 100 Hz optical noise, RF, and other disturbances. Hence, in the serial operation mode, the continuous interference and trends, that affect the output of the photodetector, cannot be rejected properly, because each channel, containing a part of the interference, is processed periodically and independently. A conventional prefiltering of the photodetector's output (in order to enhance the integral signal before the channel separation) breaks the independence between the consequent signal frames.
Replacing the serial order in optic by a parallel order in the conventional techniques leads to classical frequency-separation methods. In the parallel operation mode regime, all LEDs are activated concurrently, however with different switching rates. The further recovering of different optic channels from the composite signal (when the number of channels is small) is usually performed by means of a frequency-selective filter-bank.
It should be noted that in parallel mode regime, prefiltering as well as other conventional signal enhancing methods may generally become feasible. Nevertheless, in spite of the flexibility and universality, this regime becomes impractical when the number of wavelengths increases significantly, e.g. more than three.
For example, in the parallel operation mode regime the frequency spectrum lines (optic channels) should preferably be placed as far as possible from each other. This requirement can be hardly satisfied in practice for the case of a large number of LEDs. More specifically, a modulation-demodulation concept should meet the given technical restrictions. In practice, the modulation process is formed by triggering the LEDs at a certain rate. Thus, on the one hand, the LED triggering rate is restricted, for example, by the transient of the photodetector circuitry. On the other hand, it should be taken into account that the resulting signal may be of a rather complex form and a care should be taken that the spectra do not intersect. In particular, since the output LED signals are non-sinusoidal (i.e., nearly square waves), some harmonies of the low-rate LEDs may interfere with the fundamentals belonging to high-rate LEDs. In other words, the first and second harmonics of the slowest LED determine the interval of possible modulation rates. Hence, all the other triggering rates should be distributed properly within his restricted interval. For instance, if the lowest modulation rate is defined at 2000 Hz, then (for example for the case of ten optic channels) the remaining nine on-off LEDs' rates should be placed in the interval between 2000 Hz and its second harmonic 4000 Hz. Thus, for the nine channels the frequencies can be placed over equally spaced 200 Hz increments at points such as: 2200, 2400, . . . , 3800 Hz. It should therefore be appreciated that as a number of the optic channels increases, the frequency distance between the modulated signals decreases, thereby causing the interference problem. Thus, in the case of multiple optic channels, the parallel operation mode may result in a “crowd” signal with closely spaced and strongly coupled components.
Moreover, due to the deviations from linearity in the signal acquisition and transformation tract all harmonics may interfere in the multi-channel system, thus producing the difference and sum of the frequencies. In practice, the signal spectrum comprises not only the first and higher harmonics, but their combinations as well, i.e., multiple sub-harmonics. The latter may fall close to the fundamental frequency, thus increasing the complexity of the signal decomposition in the parallel mode.
Next, the parallel mode regime may yield a corresponding extra complexity of the circuitry of the multi-channel system, because the number of the wires coupled to the probe grows in direct proportion with the number of the optic channels.
Moreover, the concurrent run of several LEDs can cause the overloading (saturation) of the photodetector, thus restricting the signal-to-noise ratio for each particular wavelength.
Another difficulty associated with utilization of the conventional parallel mode regime in signal decomposition techniques is in the fact that the sensed optical intensity may crucially differ for different channels. Thus, the channel separation problem can be compounded by the interference between the closely spaced strong and weak signals.
Due to the aforementioned disadvantages and also other known in the art reasons, a utilization of the pure parallel mode regime in multi-channel optical systems is impractical.
Concluding the above consideration, neither pure serial operation mode regime nor parallel mode regime alone is a reasonable and practical candidate for the system that intend to use a large number of wavelengths. Hence, when a multi-channel signal acquisition is of interest, a more sophisticated technique should be advantageous.